BugMeNot is an internet service that hosts login credentials to thousands of free websites. You can use BugMeNot to by-pass free registration that is required on many sites. It can potentially help you log in without making an account at many sites. You can access these login credentials without signing up. However, to share credentials, you will have to register with BugMeNot.
Is it Useful or Not?
The problem with BugMeNot is that the service has been around so long that many websites are banned from their database. On first inspection, you might find the website useless. However, the site may be useful to some if you use their extensions which they provide to Firefox and Chrome. But in our testings, the Chrome extensions did not work.
When we tested the site, logins were not available for Google or Yahoo. We did manage to find available logins for FatWallet. The account even had $2.70 waiting in it (you will have to rack up a bit more than that if you are looking to cash out though).
But to sum it up: If you are looking to reliably log in without making an account everywhere, you will have to keep searching. If you are looking for a tool that can supplement your internet surfing, then give BugMeNot a try.